(1) Field of the Invention
The present invention relates generally to voltage control unit circuits, and more particularly, to a current sense circuit for DC-DC buck converters.
(2) Description of the Prior Art
Current sense circuits are widely used in integrated circuits. If a potentially large output current, or load current, must be driven by an on-chip switch, a current sense circuit may be used to detect the relative or absolute value of this current. The current level may be monitored to prevent damage to the switch, or to the integrated circuit from either a short circuit or a simple overloading.
A buck converter converts an input voltage to a lower output voltage. Referring now to FIG. 1 prior art, a simplified schematic of a DC/DC buck converter is illustrated. The transistor 1 works as a switch, which is driven by a high frequency pulse-width-modulated control voltage. The switch is turned on and off by the pulse-width-modulated control voltage. During the on time of the transistor switch 1 the voltage V1 is equal to Vin. Since Vin is higher than Vut the current through the diode increases linearly in correspondence to Faraday's law. When the transistor is turned off (blocking phase) the diode takes the inductor current. At this time the voltage across the inductor inverts. The voltage V1 becomes close to zero (taking into account the forward voltage drop it will be −0.7 V) and the voltage across the inductor 2 is now −Vout. The inductor current IL decreases linearly. The inductor current IL has a triangular shape, its average value is determined by the load.
As explained above it is important to sense the current via the power switch through the inductor to identify quickly overload situations. It is a challenge to the designers of such circuits to establish a current sensing circuit with a minimum efficiency loss, short settling time and good stability against process variations.
Current practice is either to measure the inductor current as a drop across a shunt resistor, or to measure it via an internal current mirror, which mirrors a fraction of the current into an internal circuit. The first method requires an additional external component and the loss of efficiency is given by the waste of power in the shunt resistor. The second method looses power, since the fraction of the mirrored inductor current is generated out of the power supply and is fed to ground. The adjustment of the operating point is currently done by using additional amplification stages.
Several prior art inventions describe current sensing circuits used in DC-DC buck converters:
U.S. Pat. No. 6,452,369 (to Lang) describes a self-oscillating buck converter including a controllable switch, a device for controlling the controllable switch, a device for sensing the output voltage of the buck converter, a device for sensing the output current of the buck converter, and a device for sensing the input voltage of the buck converter. The control device controls the controllable switch in such fashion that the output current level varies in dependence upon the output voltage and the input voltage.
U.S. Pat. No. 6,184,660 (to Hatular) discloses a battery charger IC for controlling operation of a buck converter circuit that includes a series switch and a resistor for sensing battery-charging current. The battery charger IC includes a pulse-width-modulation switch drive circuit that, during charging of the battery, supplies to the buck converter circuit with an electrical signal which repeatedly turns-on and then turns-off the series switch. The battery charger IC also includes a charging-current sense amplifier, which receives from the current-sensing resistor and amplifies an electrical signal, which represents the battery charging electrical current. The charging-current sense amplifier includes a bridge circuit to which is coupled the electrical signal received by the charging-current sense amplifier from the current-sensing resistor and an auto-zero circuit.
U.S. Pat. No. 6,381,159 (to Oknaian et al.) discloses a circuit and method for sensing the inductor current flowing to a load from a switching power supply without using a sense resistor in the path of the inductor current. In a synchronous buck converter topology, the inductor current is derived by sensing the voltage drop across the synchronous MOSFET of the half-bridge and reconstructing the current using a sample and hold technique. A ripple current synthesizer is employed to reconstruct inductor current outside the sample and hold window. A sampled product is used to update the ripple current estimator with dc information every switching cycle. The resulting voltage waveform is directly proportional to the inductor current. The inductor current synthesizer of the present invention can also be used in boost converter, flyback converter and forward converter topologies.